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I understand that a species becomes a different one slowly and in a continuous way. However, the different number of chromosomes is bugging me. Chimpanzees, for example, have 48 chromosomes whereas humans have 46.

I have few guesses which haven't satisfied me at all:

  1. The species are not that different, despite of their different number of chromosomes, and capable of breeding. The offspring is somehow healthy and is also capable of breeding. But doesn't that make the offspring sterile?

  2. The other possibility is, at some point multiple offspring in a population formed with, say, 46 chromosomes and the breeding took place among them. Even though it doesn't sound impossible to me, what are odds really?

Can someone provide an insight to a non-biologist?

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    $\begingroup$ It depends on which chromosomes are extra... For example, neanderthal and humans have different numbers, and it's the X and Y chromosome for them that differs. so only female humans which mate with a male neanderthal could have a viable chromosomal balance. The chomosomes can work in sets of three technically, they are called trivalent chromosomes. biology.stackexchange.com/questions/15727/… $\endgroup$ – com.prehensible Oct 13 '17 at 12:38
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    $\begingroup$ I'm not sure how the evolution of populations works out, but it's well known why humans have 23 chromosome pairs rather than 24 -- our chromosome 2 is the result of a fusion of two ancestral chromosomes, which remain separate in the great apes. See for example ncbi.nlm.nih.gov/pmc/articles/PMC52649 $\endgroup$ – Roland Oct 13 '17 at 17:37
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    $\begingroup$ Possible duplicate of Evolutionarily speaking, why do humans have 46 chromosomes $\endgroup$ – Fizz Oct 14 '17 at 19:54
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    $\begingroup$ You formulate guesses but what is the question you are trying to address? Does the post Evolutionarily speaking, why do humans have 46 chromosomes answers your question? I am voting as unclear for the moment. $\endgroup$ – Remi.b Oct 24 '17 at 23:24
  • $\begingroup$ I think you are overthinking the problem. Think of chromosomes as parking lots and genes as cars. Just because there are different numbers of parking lots, doesn't mean cars no longer need parking spaces - they have to park somewhere! In the specific example of humans versus chimps, two "parking lots" were joined together so there are technically less parking lots, but there is still enough room for all the cars. Perhaps I've stretched this analogy too far... $\endgroup$ – James Nov 8 '17 at 12:00
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Speciation doesn't happen from one generation to another, it is a slow and continuous process, and for each changing that is kept by natural selection, there are millions of changes that resulted in less functional phenotypes and were negatively selected, and another million of changes that were lethal hence soon aborted (e.g. a mutation that causes an embryo to die even before a fruit or animal can get any development). So it might be possible that many chromosome number changes are taking plane in many organisms, but they never will estabilsh in their populations. On the other hand, some numerical changes (and all the genetic processes that follow any change) may result in a good fit, and the individuals survive, establish and reproduce. But keep in mind that it is a slow and continuous process.

Theoretically, if among Chimpanzees, multiple offsprings outcome with 46 instead of 48 chromosomes:

  • In a theoretical scenario: They could be able to reproduce only among themselves (the 46-chromosomed-individuals) and, with time, accumulate some differences in relation to the 48-chromosomed-Chimpanzees, and them they would be named as another species.

  • In a theoretical scenario: In some conditions, they might be able even to reproduce with 48-chromosomed-Chimpanzees (e.g. if this numeric change resulted from fission of one chromosome, and when the two gametes meet, the two half chromosomes of one parental pair with the whole chromosome of the other one... That is not probable for animals, but happens a lot in plants)

  • If you are asking if they 46-chromosome chimps would crossbreed with humans... well, I believe they have accumulated too much differences in their genotypes and thus would no be compatible even if they have the same chromosome number. It happens that having the same number is not the most important factor allowing or preventing species to crossbreed. There are many barriers to hybridization, e.g. the lack of biochemical recognition between egg and sperm resulting in no embryo formation... in cases that embryo is formed, it may not develop, it may develop poorly, or it may develop but the offspring comes out compromised and live shortly. Another example that chromosome number does not prevent crosbreed is the mule, which is the offspring of a female horse (64 chromosomes) with a male donkey (62 chromosomes). That's because even having different chromosome numbers, they are still quite simmilar in a genetic perspective.

All that sayd, the odds that Chimps would loose two chromosomes, stablish in the population and reproduce with humans are really really really low... I would say that's impossible. But simmilar phenomena are possible and even quite common in plants (just so you know that it exists)

If you want to learn more about chromosome number changes, I recommend this link: https://www.ncbi.nlm.nih.gov/books/NBK21229/

(Modern Genetic Analysis. Griffiths AJF, Gelbart WM, Miller JH, et al. New York: W. H. Freeman; 1999.)

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  • $\begingroup$ I think your answer is wrong and unclear -1. Wether your answer answers the question is hard to tell as the question is unclear as well (not your fault). Here are a few critics. Evolution really takes place in a slow and continuous way this is rather wrong! What do you call numerical changes? I don't understand the sentence if among Chimpanzees, multiple offsprings outcome with 46 instead of 48 chromosomes. You seem to think that chimpanzee are humans's ancestors which is wrong. the odds are really really low for Chimpanzees the odds for what? $\endgroup$ – Remi.b Oct 24 '17 at 23:28
  • $\begingroup$ I'm surprised how poorly I seem to have expressed myself...when I say that evolution is a slow process, I mean that it does not occur from one generation to another... By numerical changes, I mean disploidy... And I don't see what in my answer made you think that I believe chimpanzees are humans's ancestors, but if you got this impression I clearly expressed myself poorly... Thus it's important to remark that humans and chimpanzees share ancestors (as we do with all the extant life, by the way). $\endgroup$ – Thai Oct 25 '17 at 0:19
  • $\begingroup$ I'll edit this answer tomorrow, thanks for pointing out so many things to improve $\endgroup$ – Thai Oct 25 '17 at 0:21
  • $\begingroup$ No worries, misunderstanding is not so uncommon. You are making a hypothetical scenario where chimps lose a chromosome pair and this feel weird to me given that the reason for why there is a mismatch between human and chimpanzees chromosome number is due to a fusion in the human lineage (Ijdo et al. 1991). Btw, welcome to Biology.SE! $\endgroup$ – Remi.b Oct 25 '17 at 0:25
  • $\begingroup$ Yes, this feels very very weird... But I got from his question that he was asking about the odds of multiple chimps loosing one pair, establishing in the community and reproducing among themselves... And this is a crazy hypothetical scenario for chimps... But things like this do happen in life... It's even very common in plants... And also, do you think humans would really match chimps if they loose 2 chromosomes? I think they now have too much differences, numeric compatibility wouldn't be enough for them to reproduce and have fertile offspring.. (Btw, thanks! =]) $\endgroup$ – Thai Oct 25 '17 at 0:39

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